Own the vendored trees by pinning them

LVGL and the vendor RV1106 SDK are what this platform is built against, and
nothing in this repo said WHICH ones. The two live outside every worktree as
shared checkouts, so "the version we build against" was whatever happened to be
on the machine.

They are pinned here now, by exact commit, with one script that obtains and
verifies them. Not copied: between them they are ~21 GB, the vendor SDK bakes
absolute paths into its host tools so it has to sit at one stable path anyway,
and a 21 GB git repo would be unusable while still not making anything
reproducible. What makes a build reproducible is knowing exactly which tree was
used, which is a commit id -- the same reasoning build/fetch-kernel-tarball.sh
already applies to the kernel, where a sha256 pin stands in for vendoring the
tarball.

Two behaviours worth stating, because both were wrong in the first draft:

- A checkout is detected by `.git` EXISTING, not by it being a directory. LVGL
  is checked out as a worktree here, where `.git` is a file.
- Local modifications are reported and are NOT a failure. The vendor SDK is
  supposed to carry them -- tools/build-firmware.sh applies our sdk-patches
  series into that tree on every build -- so a pristine checkout is the
  unusual state. Only a MISSING or DRIFTED tree fails.

Nothing is ever reset automatically: a tree off its pin is reported, because a
local change to a vendor tree is usually someone mid-debug, not something to
throw away on their behalf.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T2D2KtdgwbhbF6Mo64eUrn

README.md

@@ -58,6 +58,7 @@ scenario tests (portal, OTA apply, display + touch, watchdog).
 | `drivers/` | hardened hardware-facing drivers: HAL seams, test harnesses |
 | `kernel/` | forward-port provenance and bring-up records (`patches/` is canonical) |
 | `tools/` | `config-lint` (static memory-map gates) and dev tooling |
+| `build/vendor.manifest` | the third-party trees this platform builds against (LVGL, the vendor RV1106 SDK), pinned to exact commits; `build/fetch-vendor.sh` obtains and verifies them |
 | `docs/` | architecture, ADRs (`decisions/`), CI/CD |
 
 ## Architecture